The present invention relates in general to a an optical head for reproducing a record in an optical information recording medium, and more particularly to a technology for promoting high accuracy of tracking control of the optical head.
In order to position a condensed light spot on an information track on an optical information recording medium such as an optical disk, the focal point control is carried out in a direction perpendicular to a surface of the optical disk, while the tracking control is carried out in a direction within the surface of the optical disk. As for a method of detecting a control signal for the tracking control, some methods have been proposed. The method which is currently most widely used is a method called the push-pull method. This method is such that when a light spot which has been condensed on the disk is diffracted through the periodical guiding grooves or information pit tracks formed on the disk and both its zero-order light and its .+-. first order diffracted light are diffracted on the pupil surface of an objective lens, if the light spot is deviated from the position on the guiding grooves or between the grooves, or the position on the information pit track, since the phase of the + first order diffracted light is shifted in the direction opposite to that of the - first order diffracted light, it is detected that there arises a unbalance between the light intensity which is obtained as a result of interference between the + first order diffracted light and the zero-order light free from the phase shift and the light intensity which is obtained as a result of the interference between the - first order diffracted light and the zero-order light free from the phase shift.
These two interference intensities are detected by two division photo detector independently of each other, and a differential output therebetween is obtained in order to make it a tracking error signal. The tracking error signal is multiplied by a suitable gain and then is fed back to means for being able to move the light spot within the disk surface, such as a two-dimensional actuator or a galvanomirror each having an objective lens mounted thereon so as to form a closed loop, whereby it is possible to position the light spot on the track at all times.
Now, in the method wherein the galvanomirror which serves to move the light spot by changing an inclination of the light made incident to the objective lens is employed as the means for being able to move the light spot, the aberration may readily occur by inclining the light. For this reason, currently, there is in general employed the two-dimensional actuator which serves to move the objective lens. In this case, since the objective lens is moved so as to follow the track, thereby moving the light spot, the light spot on the two division photo detector is also moved in correspondence to a movement amount of the objective lens from an optical axis so that an offset occurs in the tracking error signal. Therefore, if the movement amount is too large, the offset can not be disregarded. As a result, there are caused reduction of an amplitude of the reproduced signal and increase of the cross talk between the adjacent tracks, and hence the signal-to-noise ratio of the reproduced signal is degraded.
The prior art for solving the above-mentioned problem is, for example, described in an article of NATIONAL TECHNICAL REPORT, Vol. 40, No. 6 (1994), pp. 771 to 778. In this technology, an objective lens, a .lambda./4 plate and a polarizing type diffraction grating are integrated in order to be mounted to a two-dimensional actuator. Then, the polarizing type diffraction grating is designed in such a way that the interference area between the + first order diffracted light of the diffracted light caused by the disk and the zero-order light, and the interference area between the - first order diffracted light of the diffracted light caused by the disk and the zero-order light are respectively diffracted at different angles of diffraction. If such a structure is adopted, the interference area between the + first order diffracted light and the zero-order light and the interference area between the - first order diffracted light and the zero-order light can be separated from each other on the detector. Therefore, if the two division photo detector is designed in such a way that even if the objective lens is moved, those light beams are not deviated from any position on the detector, it is possible to cancel the offset which is caused by movement of the light spot on the detector.
In addition, since the diffraction grating is made have the polarizing characteristics, it is made possible that the diffraction efficiency is made approximate zero when the light which is being directed to the disk permeates through the diffraction grating, while the suitable diffraction efficiency is obtained when the light reflected from the disk permeates through the diffraction grating again. If the diffraction grating of interest is the normal diffraction grating which has no polarizing characteristics, since the light which is being directed to the disk is also diffracted, the loss in the quantity of light can not be avoided. However, by making the diffraction grating of interest the polarizing type diffraction grating in such a way, only the diffraction of the necessary reflected light can be caused and hence the loss in the quality of light can be prevented.
However, in the above-mentioned prior art example, since the zero-order light which is not diffracted through the polarizing type diffraction grating is directly returned back to a semiconductor laser, only the diffracted light can be received. In this case, however, the separation of the interference area between the + first order diffracted light caused by the disk and the zero-order light from the interference area between the - first order diffracted light caused by the disk and the zero-order light becomes inconvenient when detecting a focal point deviation signal. Heretofore, the methods which have been widely used as the focal point deviation method in the optical disk head are the astigmatism method and the beam size detecting method. The former is such that the astigmatism is generated in the detected beam of light using a cylindrical lens or the like, and a four division detector is arranged in the minimum circle of confusion when the focusing state is obtained on the disk, and the differential output between the sum signal of the detector outputs in one diagonal direction thereof and the sum signal of the detector outputs in the other diagonal direction thereof is obtained. The latter is such that the differential output between the sum signals of the outputs of the two two-division photo detectors which are arranged before and after the focal point position is obtained. In this connection, in both the methods, the division directions of the division detector are determined in such a way that the disturbance which is caused when the light spot travels across the track on the disk does not get mixed with the focal point deviation signal. In such focal point deviation detection, since the detection of the intensity distribution of the light spot resulting from the focal point deviation, it is inconvenient that the light which is made incident to the detection system through the polarizing type diffraction grating is separated from the beginning.
In the above-mentioned prior art example, in order to solve that problem, the polarizing type diffraction grating is divided into a tracking signal detection area and a focal point deviation signal detection area so that the two areas correspond to the different diffraction directions, respectively. In this case, however, since the polarizing type diffraction grating becomes complicated, the manufacturing cost thereof is necessarily increased. In addition, there arises a problem that since the zero-order light is directly returned back to the semiconductor laser, the laser noises are generated and hence the reduction of quality of the reproduced signal is brought on. In addition, in recent years, the optical disks having different substrate thicknesses have been developed and are planned to come onto the market. However, the above-mentioned prior art does not make mention of interchange of such optical disks having different substrate thicknesses at all.